1 Field of Invention
The present invention relates to a novel process for the production of coated paper which has a particularly high degree of whiteness. The present invention furthermore relates to papers which are produced by this process and the printing of papers which are produced by this process.
2 Description of the Background
Paper coating slips substantially comprise a generally white pigment, a polymeric binder and additives which, for example, influence the rheological properties of the coating slip and the properties of the surface of the coated paper in the desired manner. Such additives are frequently also referred to as cobinders. By means of the binder, the pigments are fixed on the paper and the cohesiveness in the resulting coating is ensured.
Base papers acquire a smooth, uniformly white surface as a result of coating with paper coating slips. The paper coating slips additionally result in an improvement in the printability of the paper. In order to obtain optimum qualities, papers are frequently also coated two or three times, i.e. a coating slip is applied a second or a third time to a precoated paper.
The coating of paper with paper coating slips is well known nowadays, cf. for example The Essential Guide to Aqueous Coating of Paper and Board, T. W. R. Dean (ed.), published by the Paper Industry Technical Association (PITA), 1997.
One of the most important objects of coating paper with coating slips is to increase the whiteness of the paper. The object for a person skilled in the art is to provide paper having improved properties, in particular having greater whiteness, by coating uncoated paper, which is also referred to below as coating paper or base paper, or by coating precoated paper.
For this purpose, fluorescent brighteners (fluorescent or phosphorescent dyes) or optical brighteners are added to the coating slip, in particular that which is to form the top coat. Said brighteners are dye-like fluorescent compounds which absorb the short-wave, ultraviolet light invisible to the human eye and emit it again as longer-wavelength blue light, with the result that the human eye perceives a greater whiteness, so that the whiteness is increased.
The optical brighteners used in the paper industry are generally 1,3,5-triazinyl derivatives of 4,4′-diaminostilbene-2,2′-disulfonic acid, which may carry additional sulfo groups. An overview of such brighteners is to be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2000 Electronic Release, OPTICAL BRIGHTENERS—Chemistry of Technical Products. However, more recent brightener types are also suitable, for example derivatives of 4,4′-distyrylbiphenyl, as likewise described in the abovementioned Ullmann's Encyclopedia of Industrial Chemistry.
However, the use of optical brighteners in the coating slip leads to optimum success only when they are present in the final coating of the paper in an optimum structure, conformation and distribution, since, for example in the case of stilbenes, only the trans form is optically active and it only exhibits maximum fluorescence when it is distributed in monomolecular form and is fixed in a plane (K. P. Kreutzer, Grundprozesse der Papiererzeugung 2: Grenzflächenvorgänge beim Einsatz chemischer Hilfsmittel, H.-G. Völkel and R. Grenz (editors), PTS Munich, 2000, PTS manuscript: PTS-GPE—SE 2031-2). In order to achieve this, polymeric compounds which enhance the effect of the optical brightener in the coating slip and are referred to as activators or carriers are added to the paper coating slip. To date, these activators have always been mixed with the paper coating slip. An important function of the cobinders mentioned at the outset in coating slips is their brightener-activating effect. Suitable cobinders which may be used are water-soluble polymers, e.g. polyvinyl alcohol, carboxymethylcellulose, anionic or nonionic degraded starches, casein, soybean protein, water-soluble styrene/acrylate copolymers and acrylate-containing copolymers (cf. for example K. P. Kreutzer, loc. cit.).
All these compounds, which are referred to below as activators, are polymeric compounds which have the problem that they increase the viscosity of the coating slips. Consequently, an increase in the amount in which they are used in order to obtain greater whiteness is subject to narrow limits. Paper coating slips comprising polymers and copolymers which contain N-vinylformamide in the form of polymerized units, as described in the German application with the application number 100 55 592.6, are particularly effective with regard to the activation of optical brighteners.